The present invention relates generally to gas turbine engines, and, more specifically, to turbine nozzles therein.
In a gas turbine engine, air is pressurized in a compressor and mixed with fuel in a combustor for generating hot combustion gases which flow downstream through turbine stages that extract energy therefrom. The high pressure turbine disposed directly downstream of the combustor includes an annular stator nozzle which directs the combustion gases towards a corresponding row of rotor blades extending outwardly from a rotor disk.
The turbine nozzle is formed in arcuate segments for reducing thermal stress therein as the nozzle expands and contracts during operation. Each nozzle segment typically includes a pair of stator vanes fixedly joined to outer and inner arcuate band segments. Since the nozzle vanes are directly exposed to the hot combustion gases, they and their bands are commonly formed of superalloys which maintain strength at elevated temperature.
The vanes and bands are typically cast as individual pieces, assembled together, and brazed at the two roots of each vane where it is mounted in corresponding vane seats in the bands. Brazing provides a strong bond without degrading the high-strength performance of the superalloy nozzle material.
During engine operation, the nozzle is protected from the hot combustion gases by channeling a portion of compressor air inside the hollow vanes for internal cooling thereof, with the air being discharged through rows of film cooling holes extending through one or both sidewalls of the vanes. Since the vanes have airfoil configurations which taper to thin trailing edges, a row of trailing edge apertures is provided for discharging some of the cooling air through the trailing edge and cooling the thin trailing edge region of the vanes.
In one exemplary design, each vane includes a radially extending forward cavity behind the leading edge thereof, and a second radially extending aft cavity disposed at the mid-chord region of the vane between the forward cavity and the trailing edge region of the vane. The two cavities are separated by an internal imperforate bridge for isolating the two cooling circuits from each other.
The forward cavity includes an inlet through the inner band and is closed at the outer band for independently channeling cooling air therein for discharge from the film cooling holes around the leading edge region of the vane.
The aft cavity has an inlet through the outer band and is closed at the inner band for independently receiving cooling air therein which is discharged through film cooling holes of the vane sidewalls as well as through the trailing edge apertures.
Except for the corresponding cavity inlets in the opposite root ends of the vanes, the vane roots are solid or imperforate for providing a strong interconnection between the vanes and bands when brazed together. The thin trailing edge region of each vane is cooled by a row of axially extending inboard slots which join the aft cavity to corresponding ones of the trailing edge apertures.
The trailing edge apertures must be spaced inboard from the corresponding bands so that during the brazing operation the last trailing edge aperture at each end of the vane does not become plugged with braze material under capillary action. Each trailing edge aperture must therefore be sized with a sufficient flow area for channeling therethrough a corresponding portion of the cooling air for cooling the trailing edge region of the vane down to and including the braze joint interface with the bands.
A turbine nozzle of this exemplary design has been successfully used in commercial service in the United States for many years in first stage turbine nozzles of aircraft gas turbine engines. However, experience has shown that the braze joint in the trailing edge region of the vanes is subject to oxidation over extended use which limits the useful life of the turbine nozzle. Braze joint oxidation is due to the relatively high temperature experienced by the braze joint in this local region.
The cooling ability of the trailing edge apertures is at the useful limit since the size thereof is limited by maximum permitted stress during operation, and the placement of the last apertures near the outer and inner bands is limited by the manufacturing process for preventing undesirable plugging thereof by the braze material.
Accordingly, it is desired to provide a turbine nozzle having improved cooling of the braze joint in the trailing edge region of the vane for reducing oxidation thereof and improving the useful life of the turbine nozzle.
A nozzle vane includes a row of trailing edge apertures and cooperating inboard slots joined in flow communication with a mid-chord cavity. An outboard slot is spaced outwardly from a respective last one of the inboard slots, and outboard of a respective last one of the trailing edge apertures. The outboard slot extends behind a braze joint between the vane and a supporting band and is effective for backside cooling thereof.